Conventionally, switching power supply units having semiconductor devices for controlling switching power supplies are widely used as power supplies for home appliances such as home electrical products, thereby to improve power efficiency with low power consumption. In the semiconductor device, an output voltage is controlled (stabilized) by using the switching operation of a semiconductor (a switching element such as a transistor).
Particularly in recent years, in view of the prevention of global warming, attention has focused on a reduction of power consumption in a standby state of appliances such as home electrical products and thus switching power supply units achieving lower power consumption during standby are in great demand.
In order to meet the demand, a power supply system or the like is developed for properly using two switching power supply units according to an operation mode of an appliance. For example, a switching power supply unit for a main power supply is provided to supply power at rated load in a normal operation (normal mode) of the appliance, and a switching power supply unit only for a standby state is separately provided to supply power during standby in a standby operation (standby mode) of the appliance. When the appliance is in a standby state, power is supplied from the switching power supply unit only for a standby state. At rated load, power is supplied from the switching power supply for the main power supply.
However, this power supply system requires two switching power supply units (converters), increasing the cost of the overall circuit including the semiconductor device for controlling the switching power supply. Therefore, when lower cost is strongly demanded, a power supply system comprising a single switching power supply unit (converter) is used in many cases. In this case, switching power supply units of partial resonance have been frequently used in view of efficiency of power supplies and noise.
In such a semiconductor device for controlling a switching power supply, a current applied to a switching element is reduced at light load, e.g., during standby. However, it is always necessary to supply, via a transformer, the internal circuit current of the semiconductor device for controlling the switching power supply. Therefore, it is not possible to reduce a current flowing to the switching power supply, including a current flowing to the switching element, to 0 and thus a certain amount of current is applied even at no load. Hence, the switching operation of the switching element causes a loss even at no load and a lighter load causes a larger loss in the switching element. Consequently, the switching power supply decreases in power efficiency and thus it is not possible to meet the need for lower power consumption in a standby state of the power supply.
As described above, the switching power supply of partial resonance has the following problem: since the switching power supply generally has a high oscillation frequency at light load, a switching loss considerably increases and the efficiency of the power supply decreases in a standby mode.
As a solution for the low efficiency of a power supply in a standby mode (e.g., Japanese Patent Laid-Open No. 2002-315333), the following controlling technique is available: a state of a load on the secondary side of the power supply is detected by a microcomputer, the power supply shifts to a standby mode in response to the signal, and intermittent oscillation occurs due to feedback control according to a commercial frequency. In this case, in order to improve the efficiency of the power supply in a standby mode, feedback control is performed by the microcomputer as follows: when an output voltage increases to a predetermined value or higher at light load, the switching operation of a switching element is stopped, and then when the output voltage decreases to the predetermined value or lower, the switching operation of the switching element is restarted.